1. Field of the Invention
The present invention relates to a method and apparatus for measuring rigging loads, such as for the standing rigging of sail boats. More particularly, the present invention relates to a method and apparatus for measuring rigging loads utilizing strain gages.
2. Description of the Related Art
In sailboats, all rigging for supporting the mast and keeping it straight which are permanently installed and provide the means to transmit sail power to the hull are called standing rigging. The components and hardware that make up the standing rigging in a boat are designed to work with the hull to transmit wind power into forward motion. This process is accomplished most efficiently when the rigging is properly tuned to the optimum component tension.
The loading that occurs in all standing rigging can be classified as either static or dynamic. Static loads are caused by the tightening or loosening of the rigging hardware and are always present. The easiest way to increase or decrease the amount of static loading is by adjusting turnbuckles (or similar hardware). Static loads are considered to be only those caused by the tightening of standing rigging components. Allowable constant static loading does not have a negative effect in the life of components.
Dynamic loads, also referred to as xe2x80x9clive loadsxe2x80x9d are caused by everything else. The wind changes in course, wave action, sail trimming and everything else that changes during sailing create dynamic loading on the rigging. The amount of dynamic loading can be modified by removing, trimming or changing sails, or changing course. Dynamic loading, because of its cyclic nature, has a negative effect and shortens the useful life of components.
Static and dynamic loads are additive. This is a very important fact with potentially serious implications. If the level of static load on a stay or shroud is set very high by the rigging tuner, it may take very little dynamic load to exceed the breaking strength of the wire or other component. The opposite situation can also create an undesirable situation. If the amount of static load on a stay or shroud is set too low by the rigging tuner, the amplitude of the cyclic stress due to dynamic loading will be greater. This has a negative impact on rigging useful life.
It would be desirable to have a system capable of measuring tensile loads on rigging such as standing rigging on a sailboat or rigging for other purposes. Systems presently used on sailboats are generally restricted to the initial tuning of the rigging and are relatively inaccurate. It would be desirable to have a strains measuring system which can monitor dynamic stresses during operation of the sail boat so that choices can be made of the proper sails to fly for a given set of conditions under which the sailboat is operating.
U.S. Pat. No. 4,055,137, issued Oct. 25, 1977, to Motai et al., describes a vessel mooring system in which tension detectors are incorporated into a plurality of mooring devices so that the vessel is moored to the mooring facility in a well-balanced condition by adjusting the tensions exerted on the respective mooring devices by using tension signals issued from the tension detector.
U.S. Pat. No. 4,958,581, issued Sep. 25, 1990, to Denison describes a sailboard apparatus using strain gages for measuring the shearing forces between the mast and the sailboard.
U.S. Pat. No. 3,913,396, issued Oct. 21, 1975, to Elliot describes a monitoring system for moored floating vessels using tension measuring devices to send signals to a monitoring station.
U.S. Pat. No. 3,935,828, issued Feb. 3, 1976, to Pfund describes a tension sensor with a relatively rigid bow frame clipped onto a backstay. A strain gage is interposed between the frame and the backstay to sense the tension and send a signal to an indicating meter to provide an indication related to the forward thrust.
The NAVTEC NORSEMAN GIBB CATALOGUE, p. 51, describes the xe2x80x9cGrand Prix Load Pinxe2x80x9d that measures tension on the members to which it is attached. This device replaces standard clevis pins in the rigging. In order for this device to accurately measure loads in the rigging, it must be properly aligned. If the pin rotates due to vibration or other movement, it incurs a signal error equal to the cosine of the angle between the ideal alignment and the actual alignment. The xe2x80x9cGrand prix Load Pinxe2x80x9d is similar to products manufactured and sold by several manufacturers. These products are sold under names such as Load Pins, Shear Pins and Clevis Pins and are used in a multitude of machinery or equipment. Examples of such equipment are cranes, hoists, seismic restraints and towing equipment. Also shown is a Rod Cap or Eye combined with a transducer and monitored by a signal processors and backlit display units to form a xe2x80x9cGrand Prix Load Cell Systemxe2x80x9d. As above, in order for these devices to accurately measure loads in the rigging, they must be properly aligned. The inventive system is both different and unique with respect to the products described above in that: (1) by design and construction it is always aligned with the load path, thereby measuring 100% of the actual load accurately. The inventive system is also unique in that (2) in addition to providing the load measurement, it is also the means through which load adjustments are made, i.e., by turning the instrumented turnbuckle clockwise or counter-clockwise. It is (3) that the current invention is more accurate than the xe2x80x9cGrand Prix Load Pinxe2x80x9d or the transducer fitted Rod Cap or Eye for which there are no readily available published accuracy specifications (other products of similar design typically have accuracies between 0.5 and 1.0% of rated load). The current invention has a measured accuracy of 0.10% of rated load.
None of the above inventions and patents, taken either singularly or in combination, is seen to describe the instant invention as claimed. Thus, a method of monitoring/measuring rigging loads solving the aforementioned problems and achieving a high degree of accuracy is desired.
The present invention is a method and apparatus for monitoring and measuring the load in a sailboat""s rigging components. A rigging component such as a turnbuckle is modified to incorporate a sensing element, such as a strain gage. Strain gages are bonded and wired into a measuring circuit. The strain gage measures the strain under a load and generates an electrical signal that is directly proportional to the tension load. The load is then monitored and/or displayed using standard data acquisition equipment. The strain gage also identifies loose rigging such as may happen with shrouds which are subject to sudden high loading with movement of the boat and allows the operator to tighten that shroud or other piece of rigging.
According to the invention, by monitoring loads during rigging, the rigger can measure the interplay between components and adjust rigging hardware to obtain an optimally tuned rig. In addition, rigging loads can be measured while sailing or underway This information can be used by the crew to make informed decisions, such as changing or taking down sails, that can, among other things, preserve the structural integrity of the rigging equipment, the sails and ultimately the vessel.
The monitoring system, depending on its configuration, allows for two types of operation. The first mode, xe2x80x9cStatic Monitoringxe2x80x9d or xe2x80x9crigging Monitoring,xe2x80x9d allows the rigging tuner to set up and tune standing rigging to its optimum condition by measuring static loads. It requires sensors and a handheld device. The second mode, xe2x80x9cDynamic Monitoringxe2x80x9d or xe2x80x9cSailing Monitoringxe2x80x9d allows the crew to monitor loads while sailing. It requires sensors and a permanently installed display instrument. The same sensors are required for both types of monitoring, however, dynamic monitoring requires a permanently installed display or displays. In summary, static monitoring is used for rigging setup and tuning, and dynamic monitoring is used to make sailing decisions.
Static or Rigging Monitoring is used when installing the mast and setting up standing rigging at a dock or marina. The sensors are installed in the same manner an equivalent part is installed, i.e., a turnbuckle sensor is installed in the same way as all standard turnbuckle is installed. Once all rigging is in place, the rigger, boatyard personnel or owner starts tensioning all components while monitoring loads. Monitoring is done by connecting a handheld instrument to each sensor at a time and reading the display. Adjustments are performed on each component while monitoring until the desired tension or mast compression loads are obtained. The optimum tension or compression is obtained from the yacht designer or builder. In the absence of this information, about 15% of the yield strength of the rigging cable or rod is used.
Dynamic monitoring is used to make sailing decisions. Whereas static monitoring is used for rigging setup prior to sailing, dynamic monitoring is used underway to modify sailing configuration. Sailing configuration(number of sails, size, and weight) can be altered to maintain or increase sailing speed and to preserve rigging integrity, the sail, and/or vessel safety. The first requirement for dynamic monitoring is that rigging setup and static monitoring have been performed. The same sensors are used for both modes of operation. The main difference in operating modes resides in the type of display being used and the use of the monitoring information by the crew. A permanently installed display or displays are wired to the sensors to indicate total tension. Different sensors can be monitored by having several displays or one display with a selector switch. In addition to total tension, the display has a colored bar graph that changes from green to yellow to red, depending on the magnitude of the load. Operation within the green area would indicate acceptable loads, whereas, operation in the red zone would indicate dangerous tension levels.
Monitoring systems may be configured to measure one loading point, several, or all connections between rigging and hull. The most basic system consists of a turnbuckle or similar device connecting the backstay to the transom. This system would be useful for static monitoring. The most complete system would consist of one sensor for every stay and shroud and one display per sensor. This system can be used for static and dynamic monitoring.
Some standing rigging components and running rigging components, besides the strain-gaged turnbuckle, which may be similarly modified with strain sensors include tangs, backstay adjuster components, chain plates, rod rigging, shroud and stay extension, mast base load cells, traveler load cells, swage studs, and stem ball fittings.
The benefits derived from the inventive monitoring system are in the areas of safety, sailing efficiency and longer life for rigging hardware, hull and sails.
More specifically, the benefits from proper tuning of standing rigging include: (1) increased sailing efficiency, (2) prevention of high stress concentration areas on the hull or rigging resulting from uneven load sharing between stays and shrouds, (3) avoidance of excessive dynamic stresses due to loose or under tensioned rigging being suddenly loaded; this condition unnecessarily adds and accumulates fatigue stress cycles to the rigging, (4) avoidance of excessive static stresses on individual components and hull due to over tensioned components.
The benefits from monitoring while sailing include: (1) availability of quantitative information to the crew that can be used to make boat and sail handling decisions, (2) overload conditions on the rigging can be avoided or eliminated by changing or taking down sails or taking other appropriate action, (3) damage to sails can be avoided, and sail configuration and wind strength in combination with loading information may be used by trial and error, or logical thinking, to increase sailing efficiency, i.e., the conversion of wind power into forward movement.
Accordingly, it is a principal object of the invention to provide a system for measuring and monitoring load and strain in sailboat rigging.
It is another object of the invention to provide a system as above which allows for highly accurate tuning of standing rigging.
It is a further object of the invention to provide a system as above which measures dynamic loading on rigging, particularly stays and shrouds.
Still another object of the invention is to provide a system as above providing information which assists the operator in selection of sails.
It is yet another object of the invention to provide a system as above which employs instruments which provide visual warning of overstress or other condition.
Still another object of the invention is to provide an instrumented turnbuckle from which strain on the rigging at the turnbuckle may be determined.
It is an object of the invention to provide improved elements and arrangements thereof for the purposes described which is inexpensive, dependable and fully effective in accomplishing its intended purposes.